Patients that have ventilatory difficulties are often placed on a ventilator. A ventilator is a mechanical device designed to provide all or part of the work a body must produce to move gas into and out of the lungs. The ventilator delivers breathable gas to a patient and carries expired gas from the patient through a set of flexible tubes called a “patient breathing circuit”.
In many instances, the patient is connected to the patient breathing circuit via a “patient/breathing circuit interface”. The patient/breathing circuit interface may comprise of any one of a variety of devices that create a seal with the patient's face and/or head such that breathing gases emitted from the ventilator via the patient breathing circuit are provided to the patient's breathing passages and are not leaked to the atmosphere. The patient's “breathing passage” may include any passage that provides a link between the lungs of the patient and the patient/breathing circuit interface, such as the nasal passages, mouth, or throat. For example, many patient/breathing circuit interfaces comprise a facemask that covers the nasal passages and/or mouth of the patient. Other interfaces comprise a helmet that is worn by the patient and that creates a seal between the patient breathing circuit and the patient's head and/or face.
In certain settings, such as intensive care, a patient that is connected to a ventilator is able to breath with shallow breathes but is unable to complete a proper breathing cycle on his or her own, or in doing so requires an excessive breathing effort. In these instances, the ventilator provides only the necessary additional ventilatory support required by the patient to augment the breathing cycle. A patient's attempt to breathe “triggers” the ventilator and the ventilator then provides only the necessary pressure support to complete the inspiration. When the patient begins the expiratory phase of the breathing cycle the ventilator ceases the inspiratory pressure support and “cycles” the ventilator to provide an expiratory breath support.
Known ventilators and breathing circuits comprise a variety of flow and pressure sensors that produce signals to detect breathing efforts by the patient and to cause the ventilator to deliver a breath to the patient that is synchronous with those efforts. In known arrangements, the flow and/or pressure sensors are placed in the patient breathing circuit, in patient/breathing circuit interface, or in the ventilator. These sensors may include anemometers, ultrasonic, or differential-pressure flow sensors. These sensors are invariably-located on the ventilator side of the patient/breathing circuit interface. The sensor is applied whenever the patient is connected to the ventilator via a patient breathing circuit and a patient/breathing interface. However, with these arrangements, if the ventilator is delivering positive airway pressure and the mask is inadvertently pushed against the patient the sensing mechanism would identify the resulting flow or pressure change and interpret it as a patient's attempt to cycle the breath to expiration. This false identification would be due to the increased pressure in the facemask resulting from the inadvertent compression of the face mask. In another event, if a circuit leak occurs during the expiratory phase of the breath, the sensing mechanism would identify the resulting flow or pressure change and interpret it as a patient's attempt to trigger a breath. In the worst case scenario, the delivery of ventilatory assistance would then be in opposition to the patient's spontaneous breathing effort. Regardless, such a misinterpretation results in an asynchrony between the ventilator and the patient's respiratory efforts, ultimately reducing the assistance provided to the patient. While leaks can occur anywhere in the breathing circuit, patient/breathing circuit interface, and/or ventilator, a common location for leaks is between the patient/breathing circuit interface and the patient, which is typically where the facemask meets the patient's face, or where the helmet meets the patient's head. To mitigate such artifacts and to accommodate variations in patient breath sizes, frequently, care providers have to adjust the sensitivity of the trigger threshold to the patient activity and need. Therefore, it is desirable, in general, to provide an arrangement and method that provides early and accurate detection of patient triggers and cycle so that appropriate phase of ventilator support can be expediently delivered to the patient at the appropriate time. It is desirable, more specifically, to distinguish changes in flow and pressure that are caused by circuit leaks or compression of the breathing circuit resulting in false patient triggers and cycles, from actual patient efforts to breathe. Such an improved method or arrangement would promote synchronization between the ventilator and the patient, and ultimately provide improved ventilator support.
Another type or arrangement of sensors are used to count the number or the absence of breaths of non-intubated patients, such as used in conjunction with sleep apnea studies or apnea detection to guard against sudden infant deaths. Examples of these sensors include a thermister flow sensor or plethysmographic sensors to detect the excursion of the thorax. While these sensors are attached to the patient to monitor the occurrences of breathing, their responses tended to be slow, requiring greater than several tens or hundreds of milliseconds to measure the occurrence of a breath. It is recognized that delays in triggering mechanical ventilation assistance can cause detrimental increase in the patient work of breathing or in extreme breath dysynchrony may cause the ventilator to oppose the patient's breathing effort. The long time delay between detection of the patient's spontaneous breathing attempt and the application of mechanical ventilation assistance makes these apnea or breath detection sensors unsuitable to synchronize mechanical ventilation support of non-intubated patient.